brew-bundle: add --ignore-failures option

As suggested in #25053, I added a command line option to disable the newly introduced "exit on error" behavior.

```bash
$ brew bundle Brewfile --ignore-failures # Will continue on errors
```

To make it a little more useful, I added the possibility to set the option within the Brewfile itself:

```bash
$ cat Brewfile
--ignore-failures
tap my/tap
install formula
$ brew bundle  # Will continue on errors
```

I think this option is crucial because you should be able to define Brewfiles that tap into repositories (fails if already tapped) and there's no reason why the execution should be stopped if the installation of a single formula fails.